The invention relates to an alignment device for aligning a first object, which is provided with at least a first alignment mark, with respect to a second object which is provided with at least a second alignment mark, said device comprising a radiation source for supplying at least an alignment beam, a first object holder, a second object holder, an imaging system for imaging said first alignment mark and said second alignment mark onto each other, and a radiation-sensitive detection system arranged in the path of selected alignment beam portions coming from a first alignment mark and a second alignment mark onto which the first alignment mark is imaged, the output signal of the detection system being indicative of the extent to which the first and the second object are aligned with respect to each other.
The invention also relates to a lithographic apparatus for repetitively imaging a mask pattern on a substrate, in which apparatus such an alignment device for aligning a mask with respect to a substrate is used. The mask has at least a mask alignment mark and the substrate has at least a substrate alignment mark.
Aligning, for example, a mask alignment mark and a substrate alignment mark with respect to each other is understood to mean both directly and indirectly aligning these alignment marks. In the case of direct alignment, a substrate alignment mark is imaged on a mask alignment mark, or conversely, and the detection system is arranged behind the last mark. In the case of indirect alignment, both the substrate alignment mark and the mask alignment mark are imaged on different parts of a further, reference, mark, and the detection system is arranged behind the reference mark. In the latter case, the extent to which the substrate alignment mark and the mask alignment mark are aligned with respect to each other is determined by detecting to what extent both the substrate alignment mark and the mask alignment mark are aligned with respect to the reference mark.
The selected alignment beam portions are those portions of the alignment beam which are effectively used to image the first alignment mark on the second alignment mark. If the alignment marks are diffraction gratings, the selected alignment beam portions are the beam portions diffracted in given orders, for example the first orders, by the alignment marks.
U.S. Pat. No. 4,778,275 describes an optical lithographic projection apparatus for repetitive and reduced imaging of a mask pattern, for example, the pattern of an integrated circuit (IC) on a number of IC areas, or substrate fields, of the substrate. The mask and the substrate ale moved with respect to each other between two successive illuminations, for example, along two mutually perpendicular directions in a plane parallel to the substrate plane and the mask plane so as to successively image the mask pattern in all substrate fields.
Integrated circuits are manufactured by means of diffusion and masking techniques. A number of different mask patterns are consecutively imaged on one and the same location on a semiconductor substrate. Between the consecutive imaging steps on the same locations, the substrate must undergo the desired physical and chemical changes. To this end, the substrate must be removed from the apparatus after it has been illuminated with a first mask pattern and, after it has undergone the desired process steps, it must be placed in the apparatus again in the same position so as to illuminate it with a second mask pattern, and so forth. It must then be ensured that the projections of the second mask pattern and of the subsequent mask patterns are positioned accurately with respect to the substrate.
The lithographic techniques can also be used in the manufacture of other structures having detail dimensions of the order of micrometers or less, such as structures of integrated planar optical systems, magnetic heads, or structures of liquid crystalline display panels. Also in the manufacture of these structures, images of the mask pattern must be aligned very accurately with respect to a substrate.
In order to be able to realize the desired, great positioning accuracy, within several tenths of one micrometer in the apparatus according to U.S. Pat. No. 4,778,275, of the projection of the mask pattern with respect to the substrate, this apparatus comprises a device for aligning the substrate with respect to the mask pattern with which an alignment mark provided in the substrate is imaged on an alignment mark provided in the mask. If the image of the substrate alignment mark accurately coincides with the mask alignment mark, the substrate is correctly aligned with respect to the mask pattern. In the known alignment device, a HeNe laser beam is used as an alignment beam.
In connection with the increasing number of electronic components per IC and the resultant smaller dimensions of these components, increasingly stricter requirements are imposed on the accuracy with which ICs can be manufactured. This means that a mask pattern must be aligned with respect to the substrate fields with an increasing accuracy.
The alignment device described in U.S. Pat. No. 4,778,275 has hitherto worked to full satisfaction, but it is to be expected that with decreasing detail sizes, or line widths, of the IC patterns and with the use of novel technologies in IC manufacture, the alignment device may present problems relating to its reliability and accuracy. These problems relate to the symmetry of an alignment mark, required for alignment, and the symmetry of the processes to which the substrate, hence also the substrate alignment mark, is subjected. The known alignment device is only reliable if both the alignment mark and the processes are symmetrical.
In the manufacture of new-generation ICs with smaller line widths, the resolving power of the projection lens system used for the mask pattern projection must be increased, which means that the numerical aperture of this system must be increased. This means that the depth of focus of this system decreases. Since there will be some curvature of the image field at the desired relatively large image field of the projection lens system, there is substantially no tolerance for the evenness of the substrate. To maintain the desired evenness of the substrate, it may be polished, in between to illuminations of the substrate, by means of the chemical mechanical polishing (CMP) process. This polishing process is found to cause an asymmetrical distortion in a substrate alignment mark implemented as a grating. Besides the CMP process, the manufacture of ICs has also become more and more complex by the use of non-uniform etching processes and the provision of an increasing number of metal layers on the substrate. This also leads to an asymmetrical distortion of the substrate alignment mark. Moreover, these substrates, and hence the alignment marks, are coated with a number of transparent layers, such as oxide layers, nitride layers and poly layers. These layers may be deposited isotropically, but they may cause interference effects in the alignment beam affecting the alignment. Particularly the combination of asymmetry in the alignment mark and interference effects may give rise to relatively large alignment errors.